Metabolism and Survival Key Area 6b Growth Curves

By the end of this topic you should be able to: Draw a growth curve and note the lag, log/exponential, stationary and death phases on the graph Relate the phases of the growth curve to changes in the growth media State that some microorganisms can exhibit two types of metabolism – primary and secondary State when primary metabolism takes place and explain that it produces primary metabolites and energy State when secondary metabolism takes place and the secondary metabolites can confer an ecological advantage Give examples of how the secondary metabolites can confer an ecological advantage to the microorganism and how some can be beneficial to humans

Patterns of Growth

Growth Growth is the irreversible increase in dry biomass and it occurs when the rate of synthesis of organic materials is greater than the rate of breakdown Dry biomass is used instead of fresh mass as the water content of an organism can vary depending on its environment This is fine for multicellular organisms but when we are looking at unicellular organisms (bacteria, yeast) we tend to measure the increase in cell number over a period of time

Generation Time The time it takes for a unicellular organism to double in number is called its mean generation time or doubling time As microorganisms growing in culture use up the available nutrients and secrete metabolites, the organisms pattern of growth will change over a period of time

Patterns of growth Under ideal conditions, some species of bacteria are capable of doubling in number every 20 minutes. If you were trying to plot bacterial growth on normal graph paper, you would either run out of space very quickly on the x axis, or the scale would be so reduced it would make plotting or reading with any accuracy almost impossible. The solution is to use semi-logarithmic graph paper. This has been printed in a specific way to allow data which has a very wide range to be plotted. Logarithmic scale

In microbial cases, time is usually plotted as the independent variable (x axis) and logarithmic growth of bacteria is plotted as the dependent variable (y axis).

Growth curve lag phase - where microorganisms adjust to the conditions of the culture. Enzymes are induced to metabolise substrates 2. exponential (log) phase - during this phase the rate of growth is at its highest (doubling in number with each division) due to plentiful nutrients

Growth curve 3. stationary phase – occurs due to nutrients in the culture media becoming depleted and the production of toxic metabolites. Secondary metabolites are also produced (eg antibiotics). In the wild these metabolites confer an ecological advantage by allowing the micro-organisms which produce them to outcompete other micro-organisms

Growth curve 4. death phase - where lack of substrate and the toxic accumulation of metabolites causes death of cells

Constructing a growth curve The extent of growth of a microbial culture can be estimated by taking samples from the culture at certain time points and counting the number of cells present at each one. A count is also made at the time of inoculation so that the initial concentration of cells is known. Viable cell count – only living cells Total cell count – living and dead cells Note: only viable cell counts show a death phase where cell numbers are decreasing